C-Terminal Tail Polyglycylation and Polyglutamylation Alter Microtubule Mechanical Properties

Tubulin tyrosination/detyrosination regulate the affinity and sorting of intraflagellar transport trains on axonemal microtubule doublets

Cilia assembly and function rely on the bidirectional transport of components between the cell body and ciliary tip via Intraflagellar Transport (IFT) trains. Anterograde and retrograde IFT trains travel along the B- and A-tubules of microtubule doublets, respectively, ensuring smooth traffic flow. However, the mechanism underlying this segregation remains unclear. Here, we test whether tubulin detyrosination (enriched on B-tubules) and tyrosination (enriched on A-tubules) have a role in IFT logistics. We report that knockout of tubulin detyrosinase VashL in Chlamydomonas reinhardtii causes frequent IFT train stoppages and impaired ciliary growth. By reconstituting IFT train motility on de-membranated axonemes and synthetic microtubules, we show that anterograde and retrograde trains preferentially associate with detyrosinated and tyrosinated microtubules, respectively. We propose that tubulin tyrosination/detyrosination is crucial for spatial segregation and collision-free IFT train motion, highlighting the significance of the tubulin code in ciliary transport.

BdTTLL3B-mediated polyglycylation is involved in the spermatogenesis in Bactrocera dorsalis

Polyglycylation is a post-translational modification that generates glycine side chains in the C-terminal domains of both α- and β-tubulins. To date, the patterns and significance of polyglycylation across insect species remain largely unknown. The TTLL3B was thought to be a polyglycylase and be essential for polyglycylation in dipteran insects. In this study, the TTLL3B of Bactrocera dorsalis (BdTTLL3B) was identified and characterized. The BdTTLL3B expressed remarkably higher in adult males, especially in testes. The spatio-temporal patterns of polyglycylation were consistent with that of BdTTLL3B. Along with spermatogenesis, the intensity of polyglycylation was enhanced steadily and concentrated in elongated flagella. The expression of recombinant BdTTLL3B in Hela cells, which are genetically deficient in polyglycylation, catalyzed intracellular polyglycylation, validating the identity of BdTTLL3B as a polyglycylase. Knockout of BdTTLL3B significantly suppressed polyglycylation in testes and impaired male fertility, probably due to abnormal morphology of mitochondrial derivatives and over-accumulation of paracrystalline. Taken together, these findings indicated that the BdTTLL3B-mediated polyglycylation is involved in the spermatogenesis and play an important role in fertility of adult B. dorsalis. Therefore, the BdTTLL3B can be considered as a candidate target gene for the management of B. dorsalis, such as developing gene silencing/knockout-based sterile insect technology (SIT).

Role of tubulin C-terminal tail on mechanical properties of microtubule

Tubulin C-terminal tail (CTT) is a disordered segment extended from each tubulin monomer of αβ tubulin heterodimers, the building blocks of microtubules. The tubulin CTT contributes to the cellular function of microtubules such as intracellular transportation by regulating their interaction with other proteins and cell shape regulation by controlling microtubule polymerization dynamics. Although the mechanical integrity of microtubules is crucial for their functions, the role of tubulin CTT on microtubule mechanical properties has remained elusive. In this work, we investigate the role of tubulin CTTs in regulating the mechanical properties of microtubules by estimating the persistence lengths and investigating the buckling behavior of microtubules with and without CTT. We find that microtubules with intact CTTs exhibit twice the rigidity of microtubules lacking tubulin CTTs. Our study will widen the scope of altering microtubule mechanical properties for its application in nano bio-devices and lead to novel therapeutic approaches for neurodegenerative diseases with altered microtubule properties.

The Tubulin Code, from Molecules to Health and Disease

Microtubules are essential dynamic polymers composed of α/β-tubulin heterodimers. They support intracellular trafficking, cell division, cellular motility, and other essential cellular processes. In many species, both α-tubulin and β-tubulin are encoded by multiple genes with distinct expression profiles and functionality. Microtubules are further diversified through abundant posttranslational modifications, which are added and removed by a suite of enzymes to form complex, stereotyped cellular arrays. The genetic and chemical diversity of tubulin constitute a tubulin code that regulates intrinsic microtubule properties and is read by cellular effectors, such as molecular motors and microtubule-associated proteins, to provide spatial and temporal specificity to microtubules in cells. In this review, we synthesize the rapidly expanding tubulin code literature and highlight limitations and opportunities for the field. As complex microtubule arrays underlie essential physiological processes, a better understanding of how cells employ the tubulin code has important implications for human disease ranging from cancer to neurological disorders.

Ciliate cortical organization and dynamics for cell motility: Comparing ciliates and vertebrates

The generation of efficient fluid flow is crucial for organismal development and homeostasis, sexual reproduction, and motility. Multi-ciliated cells possess fields of motile cilia that beat in synchrony to propel fluid. Ciliary arrays are remarkably conserved in their organization and function. Ciliates have polarized multi-ciliary arrays (MCAs) to promote fluid flow for cell motility. The ciliate cortex is decorated with hundreds of basal bodies (BB) forming linear rows along the cell's anterior-posterior axis. BBs scaffold and position cilia to form the organized ciliary array. Nascent BBs assemble at the base of BBs. As nascent BBs mature, they integrate into the cortical BB and cytoskeletal network and nucleate their own cilium. The organization of MCAs is balanced between cortical stability and cortical dynamism. The cortical cytoskeletal network both establishes and maintains a stable organization of the MCA in the face of mechanical forces exerted by ciliary beating. At the same time, MCA organization is plastic, such that it remodels for optimal ciliary mobility during development and in response to environmental conditions. Such plasticity promotes effective feeding and ecological behavior required for these organisms. Together, these properties allow an organism to effectively sense, adapt to, and move through its environment.

Tubulin post-translational modifications in protists - Tiny models for solving big questions

Protists are an exceptionally diverse group of mostly single-celled eukaryotes. The organization of the microtubular cytoskeleton in protists from various evolutionary lineages has different levels of sophistication, from a network of microtubules (MTs) supporting intracellular trafficking as in Dictyostelium, to complex structures such as basal bodies and cilia/flagella enabling cell motility, and lineage-specific adaptations such as the ventral disc in Giardia. MTs building these diverse structures have specific properties partly due to the presence of tubulin post-translational modifications (PTMs). Among them there are highly evolutionarily conserved PTMs: acetylation, detyrosination, (poly)glutamylation and (poly)glycylation. In some protists also less common tubulin PTMs were identified, including phosphorylation, methylation, Δ2-, Δ5- of α-tubulin, polyubiquitination, sumoylation, or S-palmitoylation. Not surprisingly, several single-celled organisms become models to study tubulin PTMs, including their effect on MT properties and discovery of the modifying enzymes. Here, we briefly summarize the current knowledge on tubulin PTMs in unicellular eukaryotes and highlight key findings in protists as model organisms.